NFC: Add support for LDO voting

Vote for LDO regulator during probe() if LDO related
properties are present in the device tree otherwise
we will avoid LDO configuration as it's optional.

Configure VEN gpio low and unvote for LDO during driver unloading.

Change-Id: I5b0ccdfab2c93c0a28f2f231fd8c7ec5951ede25
Signed-off-by: Gaurav Singhal <gsinghal@codeaurora.org>